In several fields of communication there exists the need to deal at a receiver with the time-varying nature of a communications channel.
One such field of communication is in code division multiple access (CDMA). CDMA is used in the cdmaOne system (IS-95) and will be used in the so-called third generation (3G) mobile system. Each of these systems uses direct-sequence code division multiple access (DS-CDMA) in which orthogonal spreading codes are used on the down link to multiplex signals to multiple users. The spread signals of all users are combined together synchronously. The sum is scrambled by a long pseudo-noise code and transmitted from a base station. While the transmitted signals for the users within a cell are orthogonal to each other, the multi-path propagation between the base station and the users seriously disrupts the orthogonality in the signals received by the mobile users. Interference between the signals for different users gives rise to what is known as multiple access interference (MAI). The presence of MAI significantly degrades system performance. Various methods have been proposed for suppressing MAI, for example see:                Markku J Heikkila, “Interference Suppression in CDMA Down Link Through Adaptive Channel Equalisation”, IEEE Vehicular Technology Conference 1999, which proposes a receiver algorithm which performs adaptive channel equalisation by estimating the transmitted chip sequence;        Stefan Werner, Jorma Lilleberg, “Down Link Channel D-Correlation in CDMA Systems With Long Codes”, IEEE Vehicular Technology Conference 1999, which proposes linear detectors which exploit the special signal structure of the downlink transmission;        Irfan Ghauri, Dirk T M Slock, “Linear Receivers for the DS-CDMA Down Link exploiting Orthogonality of spreading Sequences”, IEEE Vehicular Technology Conference 1998, which proposes receivers which equalise for the estimated channel (based on the pilot signal) to render the user signals orthogonal, and a code matched filter is then used to cancel MAI for intra-cell users; and        Hooli, K, Latva-Aho, M and Juntti, M, “Multiple Access Interference Suppression With Linear Chip Equaliser in WCDMA Down Link Receivers”, Globecom '99, which proposes receivers which equalise the channel (on the chip level, so that the system will work with long scrambling codes) prior to de-spreading, to restore orthogonality.        
The suppression methods proposed in these papers are all based on the use of finite impulse response (FIR) equalisers.
If the input signal-to-noise ratio is high and if the channel is fixed, a FIR equaliser will usually achieve better performance than that achieved by a rake receiver. However, over a time-varying channel, where deep fading occurs the signal-to-noise ratio can be very low during the deep fading period. Adaptive algorithms do not work very well if there is a low signal-to-noise ratio. Although the periods of low signal-to-noise ratio caused by deep fading do not last long, the existence of low signal-to-noise ratio periods still degrades the convergence of adaptive algorithms. The faster the multi-path fading is, the more frequently low signal-to-noise ratio events occur, and the worse the adaptive equaliser performance becomes. Deep fading is one of the main reasons why adaptive equalisers perform so badly when used in conjunction with time-varying channels. Thus, the performance of an adaptive equaliser depends not only on the convergent speed, but also upon the instantaneous signal-to-noise ratio (rather than a mean signal-to-noise ratio).
When designing a FIR equaliser for a CDMA downlink receiver it is generally assumed that the equaliser should be as long as possible in order to produce a true inverse of the multi-path channel. Ghauri and Slock, cited above, state that “it is a well known result that longer equalizers give better results”. However we have appreciated that this assumption is based on the expectation that the true values for all weights can be obtained, whereas in a practical system the estimates for weights are noisy. Since the magnitude of every weight is different, the corresponding signal-to-noise ratio is also different. The estimation errors for small-value weights will be bigger than those for large-value weights in a noisy environment. Usually, small-value weights are also far from the center of the FIR filter. When the instantaneous signal-to-noise ratio is high, the benefits brought about by the small-value weights is bigger than the sum of the estimation errors, and hence the overall effect will be good. Otherwise, that is where the instantaneous signal-to-noise ratio is low, the small-value weights will lead to a worse result and should be removed from the equaliser. So we have appreciated that the optimum equaliser length in a time-varying noisy environment is variable. We make use of this appreciation in the most preferred embodiments of the invention, where an adaptive order equaliser is used prior to de-multiplexing received CDMA signals which have been pre-processed to improve the signal to noise ratio. Other embodiments use fixed—order adaptive equalisers similarly.